1. Field of the Invention
The present invention relates to a recycling technique for objects made of a magnesium alloy. In particular, it relates to a technique for removing the paint applied to an object made of a magnesium alloy. The present invention is applicable, for instance, to the removal of paint applied on the housing of a notebook computer, a cellular phone, etc.
2. Description of the Related Art
The housing of a mobile electronic device such as a notebook computer, a cellular phone and a PDA, should be made of a mechanically strong, heat-conductive and recyclable material. To meet these requirements, use is often made of metals rather than resins in making a housing.
Metals suitable for the housing fabrication are a magnesium alloy and an aluminum alloy, for example, with the use of which the weight reduction of the resultant product can be achieved. Of the two elements Mg and Al, magnesium has a greater specific tensile strength, equal heat-dissipating properties, and a smaller specific weight (about 70%) in comparison with aluminum. Thus, the Mg-based alloy is more advantageous for making the housing of an electronic device than the Al-based alloy.
For forming a desired component (housing of a notebook computer, for example) from a Mg alloy, die-casting or thixo molding may be employed. By these methods, however, an excessive amount of molten Mg alloy is needed. Specifically, in making a computer housing, 30˜50% of the supplied material is used for forming the desired component, while the remaining 50˜70% of the material is used for producing unnecessary parts that do not belong to the desired component. Typically, the unnecessary parts stem from the portion of the supplied material that is hardened in the sprue or runner in the molding die. To make effective use of the resources and attain cost reduction, the unnecessary parts should be recycled so that the material is used for another injection molding process.
A conventional recycling method is disclosed in Japanese patent application laid-open No. 2001-316739, for example. By the method, unnecessary portions of the hardened magnesium alloy is heated up, together with flux, to melt. Then, in the molten state, the ingredient adjustment is performed for the material. Finally, the material is allowed to cool down for solidification. Thus, a recycled magnesium alloy is obtained.
The above conventional method, however, suffers the following drawback. In general, a component of a magnesium alloy is painted for e.g. decorative purposes, and the paint often contains acrylic resin or urethane resin. Under such circumstances, an unacceptable amount of organic gas may be produced when the acrylic resin or urethane resin of the paint is burnt in heating up the Mg-alloy component for recycling. In addition, titanium contained in the applied paint may contaminate the molten alloy.
To overcome the above problems, the applied paint needs to be removed before the Mg-alloy component is heated up for recycling. For the paint removal, use may be made of a wet blasting technique as disclosed in Japanese patent application laid-open No. 2000-263443, for example. By the wet blast, inorganic particles (made of e.g. alumina) are caused to impinge on the painted Mg-alloy component by the flow of the water jet, so as to scrape the paint layer off the Mg-alloy component. In this manner, however, it is often difficult to completely remove the paint from the Mg-alloy component since the abrasive particles may fail to reach the paint material in the tiny pits in the component's surface.
Another known technique for removing the paint applied on a Mg-alloy component utilizes an alkali parting agent. By this method, the painted Mg-alloy component is immersed in the parting agent so that the paint layer swells by absorption of the agent. Thereafter, the paint layer can be washed away from the component's surface by running water, for example. Unfavorably, it often takes a long time, say two hours, to complete this process since the swelling of the entire paint layer (typically consisting of a lower layer and an upper layer) does not take place quickly.